694 PART IV. THE PHYSIOLOGY OF PLANTS. 



When, as is usually the case, the liquid with which the absorbent 

 cells come into relation, is a solution of a number of different sub- 

 stances, these different substances are not all absorbed in the same 

 proportion. Supposing the absorbing organ to be equally poor in 

 all these substances. Then, in the first instance, the substances 

 would be absorbed in proportion to their diffusibility, the most 

 diffusible being absorbed the most rapidly. Subsequently, how- 

 ever, the relative amount of substances absorbed would be de- 

 termined by the extent to which, after absorption, they severally 

 were chemically altered in the metabolism of the plant. For when 

 a substance is chemically altered in the plant, it ceases to exist, 

 as such, in the sap of the cells ; hence, when a substance is 

 constantly being decomposed in the plant, it can be also con- 

 tantly absorbed. When, on the other hand, an absorbed sub- 

 stance does not undergo chemical change in the plant, it ac- 

 cumulates in the sap of the cells, and consequently the absorp- 

 tion of it from without eventually ceases. 



This point can be determined with regard, at least, to the 

 mineral substances which a plant absorbs, by an analysis of the 

 ash which is left behind on incinerating the plant, that is, on 

 burning away the combustible organic matter. The ash will be 

 found to include all the mineral elements present in the soil or 

 water from which absorption is taking place, in so far as those 

 elements are present in the form of compounds which are soluble 

 and diffusible. The general constitution of the ash depends upon 

 the chemical nature of the soil, and upon the physical properties 

 of the substances which comprise it, as is proved by the fact that 

 the constitution of the ash of any given plant varies with the soil 

 in which it grows. But the relative proportion of the various 

 chemical elements in the ash depends upon the capacity of the 

 plant to chemically alter the various compounds which it absorbs, 

 as is proved by the fact that the constitution of the ash varies 

 widely in plants of different kinds grown in the same soil ; it is 

 the expression of the specific metabolic properties of the plant, or, 

 in other words, of the specific absorbent capacity of the plant. And 

 not only has each plant a specific absorbent capacity, but this 

 varies at different periods in its development, in harmony with 

 the variation which takes place in its metabolic processes. 



A good illustration of the different specific absorbent capacities of plants is 

 afforded by a comparison of the amount of silica in the ash of different plants. 

 Thus, an analysis of meadow-hay and of pea-straw, grown under the same 



